The present invention generally relates to a process and an apparatus for transfering a gas into a liquid body, in particular, the invention relates to aeration of industrial and municipal wastewater in the activated sludge process.
The activated sludge process for wastewater treatment comprises stablizing the wastewater impurities by means of aerobic biological growth. Adequate quantity of oxygen must be supplied to the microbial cells in order to keep them continuously digesting the organic impurities. At the same time, the treated liquid must be kept in sufficient mixing throughout the reactor so that the suspended biomass will not settle out from the reacting liquid. Therefore a properly designed aeration system must satisfy two process requirements, namely supplying desired amount of oxygen to the liquid and providing overall liquid mixing in the reactor.
It is well known that the aeration of activated sludge can be accomplished by mechanical surface aerators. A surface aerator contains a rotational shaft carrying a partially submerged rotor with a number of agitating blades. Said blades continously motivate the surface liquid and spray it in the gas space to form a jet layer. The jet layer, which is constantly renewed by the rotor, contacts with the oxygen contained gas and thereby results in oxygen dissolution in the liquid. As the jet impinges the liquid surface it entrains a quantity of gas pockets in the liquid surface layer, while the impact of the impingement produces surface turbulence which promotes a intimate contact between the liquid and the entrained gas bubbles. In general, the quantity of the gas bubbles are limited by the jet impingement which is insufficient to provide a optimum mass transfer efficiency. Therefore the performance of surface aerators can be further improved by introducing additional gas to the surface layer of the liquid.
Because a surface aerator has a relatively simple mechanical arrangement, it enjoys a growing popularity in the modern wastewater industry. However there is also a fundamental shortcoming: The liquid circulation is limited to the surface layer and the aeration basin has inhomogeneous oxygen distribution and insufficient mixing in the bottom portion. Under this situation, a part of the solids will settle out and accumulate on the basin bottom. The settlement of solid is very undesirable from the standpoint of activated sludge process for it develops an anaerobic reaction condition. The generally known solution to this problem is to add a bottom mixer to the common shaft of the surface aerator. However, the additional bottom mixers are disposed close to the bottom of the basin, it can only perform bottom mixing but not oxygen transfer, for no gas bubbles can penetrate to the bottom section. Therefore the power consumed by the bottom mixer does not contribute additional oxygen transfer and the overall mass transfer efficiency is sharply reduced. From the standpoint of oxygen transfer it is beneficial to introduce extra gas to the bottom section of the reactor so that the mixing power of the bottom mixer can be utilized for oxygen transfer. A part of the introduced gas will be dissolved in the bottom mixing zone and the undissolved gas bubbles will rise to the surface layer of the liquid to enhance the mass transfer of the surface aerator. However, the operation of injecting gas into the liquid bottom section can not be carried out without additional complicated mechanical system in the conventional manner. In order to overcome the hydrostatic pressure, the injected gas must be pressurized by a blower such as described in U.S. Pat. No. 3,827,679. The extra costs of the blower and the gas pipe line in this patented system must be spent in addition to the surface aeration reactor and therefore the advantage of the high efficiency of oxygen transfer is counter-balanced by the high capital costs and the complicated mechanical system.
The ideal system for enhancing the surface aeration and providing bottom mixing would include a novel means of gas injection which requires no additional prime mover. The present invention provides an aeration method and apparatus which have the features of the above stated ideal system.